Hanging pawns on the board. This typical pawn structure is probably the second most important in chess, after the isolated queen's pawn.
13. f1e1 This doesn't seem like the scariest of moves, but the problem for White in this line is that the e3-e4 advance isn't that promising and patience is needed. 13... c6c5 A new and natural move. 14. c4d5  14... e6d5  15. d4c5 